Twenty-four patients with abnormal nipple discharge were admitted to the Second Department of Surgery at Tokyo Women's Medical College between January 1988 and December 1988. In all of these cases, dilatation of the lactiferous duct through to the nipple was found by ultrasonic examination. We histopathologically examined 10 of these patients in whom a tumor with a dilated lactiferous duct was detected ultrasonographically, and compared the diagnostic values of ultrasonography with those of mammography, galactography and cytology. We histopathologically examined all cases after locating the tumors by ultrasonography. The lesions were diagnosed as intraductal papilloma in 4 of the 10 cases, intraductal papillomatosis in 1 case, noninvasive carcinoma in 1 case, papillotubular carcinoma in 3 cases and solid-tubular carcinoma in 1 case. The sensitivities of ultrasonography, mammography, galactography and cytology were 80%, 0%, 40% and 60% respectively in the 5 patients with benign tumors, and 100%, 60%, 60% and 60% in the 5 patients with carcinomas. We paid special attention to the dilated lactiferous duct that passed through to the nipple in these patients and found that the ultrasonographic images of the lactiferous duct, the intraductal tumor and the peripheral tumor were useful diagnostic tools to differentiate cases of breast cancer from benign tumors. Ultrasonographically it is possible not only to locate these growths rapidly and accurately, but also to differentiate breast cancers from benign tumors. This is important when trying to determine the proper course of treatment.